Gas turbine engines for short-life expendable applications commonly employ rolling element bearings to journal rotating engine parts. Adequate lubrication of such bearings is essential to meeting designed life and reliability requirements. Long-life non-expendable engines use recirculating oil lubrication systems to secure optimal bearing life. However, such recirculating oil systems are not suitable for expendable engines due to their complexity, weight and cost.
Expendable short-life engines also have design requirements that include maintenance-free long-term storage without servicing prior to use. Conventional recirculating oil lubrication systems generally exhibit some degree of oil leakage with long-term storage or when stored in non-upright attitudes.
One example of a lubrication system for expendable engines that does not incur the limitations of complexity, weight, cost, leakage and restricted storage conditions of recirculating oil lubrication systems is a so-called “constant loss” non-recirculating lubrication system. It comprises an oil reservoir and a simple delivery mechanism. The delivery mechanism supplies oil to the bearings that flows through them and then through the engine flow path. There is no recirculation of the supplied oil so that lubrication only continues as long as the reservoir can deliver oil. The advantages of this system comprise its simplicity the excellent lubrication qualities of the oil that it delivers. The limited operating time restricted by the size of the reservoir and the potential for reservoir leakage offset these advantages.
Another example of a lubrication system for expendable engines is a fuel lubricant non-recirculating lubrication system. With this system, fuel supplies and lubricates the bearings and then passes through the engine flow path. This system has the advantages of simplicity and elimination of possible lubricant leakage. However, the poor lubrication qualities of the fuel offset these advantages.